1. Field of Invention
The present invention relates in general to delta-sigma modulators and in particular, to noise shapers with shared and independent filters and multiple quantizers and data converters and methods using the same.
2. Background of Invention
Delta-sigma modulators are particularly useful in digital to analog and analog to digital converters (DACs and ADCs). Using oversampling, the delta-sigma modulator spreads the quantization noise power across the oversampling frequency band, which is typically much greater than the input signal bandwidth. Additionally, the delta sigma modulator performs noise shaping by acting as a highpass filter to the noise; most of the quantization noise power is thereby shifted out of the signal band.
The typical delta sigma modulator in an ADC includes an input summer which sums the analog input signal with negative feedback, an analog linear (loop) filter, a quantizer and a feedback loop with a digital to analog converter (feedback DAC) coupling the quantizer output and the inverting input of the input summer. A delta-sigma DAC is similar, with a digital input summer, a digital linear filter, a digital feedback loop, a quantizer and an output DAC at the modulator output. In a first order modulator, the linear filter comprises a single integrator stage; the filter in higher order modulators normally includes a cascade of a corresponding number of integrator stages. Higher-order modulators have improved quantization noise transfer characteristics over modulators of lower order, but stability becomes a more critical design factor as the order increases. For a given topology, the quantizer may be either a one-bit or a multiple-bit quantizer.
The feedback DACs in multi-bit delta-sigma ADCs, as well as the output DACs in multi-bit delta-sigma DACS, are typically constructed from weighted conversion elements. Each conversion element converts one digital bit into a weighted-step analog voltage or current. The currents or voltages generated by the weighted conversion elements for the digital word being converted are then summed to generate the analog output signal. Mismatch between conversion elements, however, causes the weighted steps of current or voltage to deviate from their ideal weighted-step values. Element mismatch results in mismatch noise and distortion in the output signal. Consequently, dynamic element matching (DEM) circuitry is normally included at the DAC inputs which spreads the mismatch noise across the analog output signal band.
For example, a number of well-known DEM designs including barrel-shifting, individual level averaging, butterfly routing, and data weighted averaging, exist. DEM circuits however do have significant drawbacks. Also, in multiple-bit modulators the DEM circuitry is relatively large, especially in high voltage ADCs requiring a large fabrication geometry. In addition, a tendency for the DEM circuit to become tonal exists, and the DEM circuit is typically a low order, delta-sigma modulator.
Hence what is needed are techniques which address the problem of mismatch between data conversion elements in DACs and ADCs. Such techniques should, for example, eliminate or minimize the DEM circuitry required in the given DAC or ADC.
The principles of the present invention generally apply to noise shapers with multiple quantizers and shared and independent filter functions. In one representative embodiment, a noise shaper is disclosed including first and second quantizers and first and second feedback paths each providing feedback from a corresponding quantizer output. A loop filter system implements a plurality of transfer functions including a first non-zero transfer function between the first feedback path and an input the first quantizer, a second non-zero transfer function between the first feedback path and an input of the second quantizer, a third non-zero transfer function between the second feedback path and the input of the first quantizer and a fourth non-zero transfer between the second feedback path and the input the second quantizer.
Noise shapers embodying the inventive principles have substantial advantages over the prior art. For example, modulators with both shared and independent filter stages and multiple-quantizers allow for the characterization of both global noise shaping across all modulator outputs and local noise shaping at individual modulator outputs. Global noise shaping is the ability of the delta-sigma modulator to shape the total quantization noise spectrum generated by the sum of the output spectrums of the multiple quantizers. Local noise shaping is the ability of the delta-sigma modulator to shape the spectrum of the difference of the spectrums output from the multiple quantizers. In other words, global noise shaping characterizes the overall shaping modulator output spectrum and local noise shaping allows the difference noise spectrum exposed to mismatch in the following conversion elements to be shaped. Generally, an improvement in global noise shaping results in a reduction in the local noise shaping capability, and vice versa. Furthermore, the inventive principles may be applied to a number of different modulator topologies, including feedforward, feedback, and combination feedforwardxe2x80x94feedback topologies.